This invention relates to an apparatus for indirect measurement of the level of liquid metals or other electroconductive liquids, and particularly to an apparatus for inductive continuous measurement of the level of a liquid which has automatic temperature compensation.
Liquid metal, particularly liquid sodium, is widely employed as a reactor coolant in a fast breeder reactor and various research facilities related thereto due to the high fluidity and excellent thermal conductivity of such metal, and the measurement and control of the level of the liquid sodium is an important subject from the viewpoint of process control and also of the safety of such facilities.
Various liquid level detectors based on various physical principles have heretofore been developed. Among these prior art detectors, the one considered most promising and actually employed in the above-mentioned field of nuclear facilities is an inductive continuous liquid level detector, which is based on the electroconductivity of liquid metal and functions as follows. A probe provided with a primary coil and a secondary coil in a non-magnetic sheath is dipped in the liquid metal, and the primary coil is supplied with an alternating current from an oscillator to generate induced magnetic flux. Since such magnetic flux gives rise to an eddy current in the liquid metal which has the effect of cancelling the magnetic flux, a change in the liquid level results therefore in a change in a mutual inductance between the primary and secondary coils and thus in the voltage induced in the secondary coil. The detection of the liquid level can therefore be achieved by the measurement of the induced voltage in the secondary coil. An inductive liquid level detector of this type is advantageous in that the measurement can be indirectly achieved without direct contact with the liquid metal since the coils are placed in the non-magnetic sheath, and in the possibility of easy repair since the coils are easily extractable from the sheath. However, the temperatures of liquid metal and gas in the reactor tank are subjected to variations over a rather wide range and are usually mutually different. Due to such temperature variations in liquid metal and gas, there result variations not only in the physical properties of the materials constituting the liquid level detector such as the resistance of coils or the magnetic properties and resistivity of metallic material constituting the sheath, but also the physical properties, particularly the electroconductivity, of the liquid metal itself, thereby rendering exact level detection difficult. Although various methods have heretofore been proposed, no satisfactory solution has been reached for compensating for such errors due to temperature variations. In addition the deposition of evaporated metal on the external surface of the sheath after prolonged operation can be a cause of error in the detection.
As an example of prior art liquid level detectors with a temperature compensation feature, there is a detector consisting of a level detecting probe and a temperature compensating probe both placed in parallel in a tank containing liquid metal as disclosed in the Japanese Patent Official Gazette, Laid-Open Specification No. 48-43367. Both of these probes are provided with a primary and a secondary coils arranged in a sheath, and one of these probes is used as the level detecting probe and the other as the temperature compensating probe. The latter is provided, around said coils, with a `substitute` metal having the same magnetic permeability as that of the subject liquid metal. The primary coils in both probes are supplied with an output current from an oscillator, and the difference between the voltages induced in the secondary coils of both probes is detected. In such a liquid level detector, the voltage induced in the secondary coil of the temperature compensating probe represents, due to the presence of the substitute metal therearound, a state where the coils are constantly and the temperature compensating probe completely immersed within the liquid metal and does not respond to the variation of the liquid level. Thus, both probes are always under completely identical temperature conditions even when there occurs a change in a temperature distribution resulting from the temperature change as well as from a level change, and any temperature-dependent change in the output voltage of the temperature compensating probe should be equal to any temperature-dependent component of the change in the output voltage of the secondary coil of the level detecting probe, and the level detection can therefore be achieved more accurately by determining the difference between these two induced voltages. Such a liquid level detector, however, inevitably has certain fundamental drawbacks. Firstly, there exists no substitute metal the physical properties of which change in an exactly same manner as those of the subject liquid metal, and for this reason the temperature compensation is merely an approximate and indirect one involving inevitable errors. Secondly, because there is usually a considerable difference between the temperatures of the liquid metal and of the gas in the tank, the probe itself likewise shows a temperature difference between the part thereof immersed in the liquid metal and the remaining non-immersed part. Such a temperature difference as well as the variation of the temperatures of the liquid metal and gas give rise to a change in the magnetic permeability as well as other physical properties of the substitute metal, thus affecting the output voltage of the temperature compensating probe and leading to an error. Thirdly, the presence of two probes which have to be placed in the tank gives rise to a limitation in the tank structure and space required therefor.
In liquid level detection by an inductive level detector, the principal temperature-dependent factors leading to an error can be summarized as follows.
(1) The change of resistance of the coils due to the temperature variation of the liquid metal:
(2) The change of the resistance of coils resulting from the temperature difference between the liquid metal and the gas in the reactor tank:
(3) The change of physical properties, particularly electroconductivity, of the liquid metal itself due to the temperature variation thereof: and
(4) The change of electromagnetic properties of the materials constituting the probes due to the temperature variation of the liquid metal.
A complete compensation for such errors has not been achieved in the prior art, primarily due to incomplete analysis of the causes leading to the temperature-dependent error.